Ordinary examples of a transmission line used to transmit high-frequency electrical signals of the related art which uses a frequency band of 20 GHz or more include a microstrip (MSW)-type transmission line that is called a microstrip line which is provided with a signal electrode used to transmit high-frequency electrical signals on a front surface (one principal surface) of a dielectric substrate and includes a GND electrode (ground electrode) formed on a rear surface (the other principal surface) and a coplanar (CPW)-type transmission line that is called a coplanar line which includes a signal electrode used to transmit high-frequency electrical signals and a GND electrode (ground electrode) formed on a front surface (one principal surface) of a dielectric substrate (refer to Japanese Unexamined Patent Application Publication No. 2005-73225 and Japanese Unexamined Patent Application Publication No. 2005-236826).
However, the microstrip (MSW)-type transmission line has a problem in that, since there is a limitation in the width and thickness of the GND electrode due to the thickness and permittivity of the substrate, and it is difficult to design the connection from other electrode patterns to the GND electrode, there is a limitation in the electrical connection with other components.
In addition, the coplanar (CPW)-type transmission line includes the signal electrode and the GND electrode formed on the front surface of the substrate, and therefore the coplanar-type transmission line can be easily connected with other components, and the impedance can be controlled using a gap (interval) between the signal electrode and the GND electrode, which leads to an advantage of a small limitation in design.
When the coplanar (CPW)-type transmission line is put into actual use, the substrate needs to be accommodated in a metal box for electromagnetic shield or protection. In this case, the bottom surface of the substrate being accommodated serves as a ground, and a grounded coplanar (GCPW)-type transmission line called a grounded coplanar line is formed.
In the GCPW-type transmission line, the influence of a metallic wall surface becomes significant, and a deterioration phenomenon occurs in which a dip-shaped (S21) loss of the transmission characteristics due to resonance in an operation frequency increases. Therefore, in order to prevent the occurrence of the above-described deterioration in an operation frequency range, different solutions have been proposed, including optimizing the location of the metallic wall, providing a number of via holes that electrically connect the ground surface of the coplanar (GCPW)-type transmission line and the ground surface on the bottom surface of the substrate, and the like.